264 THE VARIATION, RACES AND MIGRATIONS OF THE MACKEREL. 



existence. Spots of this kind can only be found in fish whose 

 transverse bars have departed from their primitive relation to the 

 myotomes, and in which the primitive symmetry has been replaced 

 by a new symmetry, due to the breaking up of the primary bars and 

 the anastomosis of the fragments of one bar with those of its neigh- 

 bours. Some of these fragments remain permanently isolated, and 

 constitute the intermediate spots under discussion ; while to right 

 and left of them are seen the new or " secondary " bars which have 

 resulted from the union of the other fragments of the primary bars. 



Thus the presence of intermediate spots is in itself evidence of the 

 transformation of the primary bars, and the frequency of these spots 

 in races of fish may be expected to vary according as the process 

 of transformation has proceeded to a greater or less extent. 



A comparison of the markings of the different species of the genus 

 Scomher shows, indeed, that they are subject to extraordinary modifica- 

 tions. Thus, according to Giinther's catalogue (vol. ii,, pp. 357-362), 

 the stripes are transverse in IS. scomher, 2Jncumatophorus, Janesaha, and 

 iapcinoccphahis, irregularly reticulated in S. colias, longitudinal in 

 >S'. chrysozomis, replaced by longitudinal rows of spots in S. moluccensis, 

 and of indistinct dots in S. microlcpidotus. Why the transverse stripes 

 should be retained in one species and replaced by longitudinal stripes in 

 another, I am unable at present to say ; but it is evident that the 

 markings of the mackerel tribe are, so to speak, in a state of very 

 unstable equilibrium, and susceptible of considerable modification, a 

 fact which should render them of much service in the investigation of 

 racial differences in the more primitive species. To this group the 

 common mackerel belongs, owing to its retention of the transverse 

 stripes, modified though these are in a variety of ways. 



The variations in the frequency and numbers of the intermediate 

 spots are given in Table D (p. 292). 



The number of fish exhibiting one or more spots (which for brevity 

 will be termed "spotty fish") is seen, as a rule, to be but a small 

 proportion of the whole ; but in this respect there is a striking contrast 

 between the European and the American mackerel. In European 

 samples the number of spotty fish is usually about 25 % of the whole, 

 varying from T^/o to 29%; but in the American sample the spotty 

 fish predominate, and amount to 66 % of the total number. 



Similarly the total number of spots for every hundred fish is usually 

 about 30 in European samples, varying from 8 to 57, but attains the 

 extraordinary total of 215 in the American sample. The table giving 

 the frequency of the different numbers of spots shows that the high 

 American total is not due to the accidental inclusion of some one 

 or two very spotty fisli, but is due to a regular and extensive variation 



